Communication channels, such as wireless channels and other channels carrying digital communication, are subject to noise. The noise can be difficult to characterize, as it depends on the environment of the communication system. Noise bursts can corrupt large parts of a message as it may disturb a channel for a relatively long time. The noise bursts can be widely distributed geographically and can also vary in duration and noise level.
Typically messages are encapsulated in a frame structure and transmitted in sequential frames over a down channel from the transmitting unit to a receiving unit. In some transmission systems, in order for the transmitting unit to have assurance that the message is received correctly, the receiving unit transmits some form of acknowledgment signal to the transmitting unit over an up channel.
It is known to use automatic repeat requests (ARQ), possibly combined with forward error correction, to cause uncorrectable frames to be retransmitted until they are received correctly, or until a maximum permitted number of repeat requests is reached.
The article “A Class of Adaptive Hybrid ARQ Schemes for Wireless Links” Sunghyun Choi, FEEE Transactions on Vehicular Technology, Vol. 50, No. 3, May 2001, discusses three correction schemes including a system using an adaptive hybrid forward error correction (FEC) and ARQ using Reed Solomon code. The paper describes the selective-repeat ARQ in which the sender transmits frames without waiting for an acknowledgement (ACK) or negative acknowledgement (NAK) of frames already transmitted. this version of ARQ has better throughput than the “stop-and-wait” or the “go-back-N” versions. If the receiver is unable to correct errors using the FEC, it sends a NAK signal to the transmitter initiating re-transmission of the corrupted frame or code segment depending on the specific ARQ implementation. The NAK frame includes four bytes, the first two bytes being used to identify the frame number, the third identifying (a) whether the response is ACK or NAK and (b) adapted code rate, and the last being a check-sum. In addition, if neither ACK/NAK is not received by the transmitter within a timeout interval, the frame is retransmitted. The timeout is based on the round trip delay.
The current forms of ARQ can be effective against some forms of noise, but particularly where the noise is of long duration, the “fixed-delay” ARQ may suffer interference when the noise is still present in the channel. If the number of ARQ repeats does not exceed the maximum permitted number, it may still be possible for a repeat ARQ to be sent to request the retransmission. However, for each retransmission the latency is increased with the round trip delay and the throughput of the system can drop significantly. This may be unacceptable where latency and throughput are important. The automatic retransmission of packets when an ACK is not received can reduce the effective bandwidth of the channel by causing the unnecessary retransmission of frames which have been correctly received and only the ACK signal has been corrupted.